This application claims the benefit of priority to German Application No. 19856367.1, filed Dec. 7, 1998, which is herein incorporated by reference.
The invention relates to a process for cleaning the exhaust gas of an internal combustion engine with lambda control and trim regulation, in which a lambda probe is used with a controller.
It is known to use a three-way catalyst in the exhaust gas tract of an internal combustion engine to clean the exhaust gas. It is also known to use provide a lambda probe whose output signal is dependent upon the residual oxygen content in the raw exhaust gas upstream from the catalyst. It is believed that the residual oxygen content in the raw exhaust gas depends upon the fuel/air mixture dispersed in the internal combustion engine. It is known that in case of excess fuel (rich mixture), the residual oxygen content in the raw exhaust gas is lower, and in case of excess air (lean mixture), the residual oxygen content in the raw exhaust gas is higher.
There are also known broad-band lambda probes that are capable of outputting signals corresponding to a lambda range (0.7 to 4) in a linear fashion.
When evaluating the operating characteristics of the internal combustion engines, it is known to correlate the output signal from the lambda probe to a lambda value. It is also known that it is advantageous to correlate the lambda value of an exhaust gas to the actual lambda value of the exhaust gas- for example, when a three-way catalyst shows optimum catalytic properties at lambda=1, an output signal corresponding to lambda=1 should correspond to a condition where the lambda of the exhaust gas is equal to 1.
It is believed that the static and dynamic properties of the lambda probe upstream from the three-way catalyst are varied by aging and poisoning. It is known that the output signal which corresponds to lambda=1 differs from the actual output signal which corresponds to lambda=1. In order compensate for this differing output signal, it is well known to dispose an additional lambda probe downstream from the three-way catalyst. The additional lambda probe is used to monitor catalytic conversion by the three-way catalyst, for example, and therefore permits control of the fuel/air mixture by correcting the signal level associated with lambda=1. The use of the additional lambda probe allows optimum catalytic conversion to be sustained. This process is referred to as xe2x80x9cguidingxe2x80x9d or xe2x80x9ctrimming correction.xe2x80x9d It is known to use a measuring pickup which detects a pollutant concentration with a known correlation with the lambda value of the exhaust gas, (e.g., the NOx concentration).
It is known to use a controller in conjunction with a broad-band lambda probe. The controller actuates the broad-band probe and determines a measurement signal from a raw signal. The controller and a circuit therein may be subject to large temperature variations. Additionally, it is known that in order to operate the internal combustion engine within a range of acceptable lambda values such that optimum catalytic conversion occurs (referred to as a xe2x80x9clambda windowxe2x80x9d), it is necessary to precisely determine the measurement signal from the raw signal. In order to compensate for any inaccuracies due to the large temperature variations, it is known to operate the controller in a test mode in order to re-calibrate the controller. The re-calibration of the controller is believed to compensate for errors caused by the temperature variations. It is known that a time required for re-calibration depends on the raw signal, which is dependent on a phase of operation of the internal combustion engine. For this reason, it is desirable to operate the controller in test mode during a phase of operation which allows re-calibration during a short time interval. It is believed that the time interval is sufficiently short ensuring an idling phase of the internal combustion engine- when lambda=1.
The present invention provides a process for cleaning the exhaust gas of an internal combustion engine with a catalyst showing three-way properties disposed in the exhaust gas, and a lambda probe arranged upstream from the catalyst. The lambda probe is connected with a controller which actuates the lambda probe in order to form a measurement signal from the raw signal present at the raw signal output of the lambda probe. The regulation of the operation of the internal combustion engine is performed such that the lambda value of the raw exhaust gas assumes predetermined values at the lambda probe, a certain signal level of the measurement signal being associated with lambda=1. In a trimming adjuster, the concentration of an exhaust gas component downstream of the catalyst showing three-way properties is measured by means of an additional measuring pickup and a set value dependent thereon is formed with which the signal level of the measurement signal associated with lambda=1 is corrected. In an offset determination, an actual value of an additive measurement signal falsification developing during the formation of a measurement signal in the controller is corrected by switching the controller to a test mode in predetermined states of operation of the internal combustion engine, by determining the actual value. The actual value of the measurement signal falsification is compensated in the formation of the measurement signal. After an offset determination of the actual value of the measurement signal falsification, the actual set value of the trimming adjustment is varied to an appropriate degree contrariwise to the variation of the actual value.
The present invention provides for improved cleaning of an exhaust gas in an internal combustion engine such that a lambda range which corresponds to optimum catalytic conversion can be accurately maintained.
The present invention sets out from the knowledge that the largely constant active trimming adjustment also compensates for errors due to temperature or component inaccuracies since the set value of the trimming adjuster is adapted over a relatively long period of time such that the signal of the lambda probe downstream from the catalyst shows a value corresponding to lambda=1. If now an offset determination for the controller of the lambda probe that is ahead of the catalyst is performed, the actual value of the measurement signal falsification is compensated in the formation of the measurement signal, so that the shift of the signal level of the lambda probe ahead of the catalyst, that is caused by the set value of the trimming adjustment, is no longer correct. Only with a gradual adaptation brought about by the adaptation of the set value of the trimming adjustment will this error disappear again, and the operation of the internal combustion engine again approaches the lambda value best for the catalyst action, from which it had departed due to the abrupt change in the actual value of the signal falsification after the offset determination. To prevent this, according to the invention, after the offset has been determined the set value of the trimming adjustment is varied contrariwise to the variation of the actual value of the falsification of the measurement signal. That is to say, depending on the actual value of the measurement signal falsification, the set value of the trimming adjustment is permanently offset by the corresponding amount, or the initial value of a trimming adjuster embodied as a proportional integral regulator is changed one time after each offset determination. This contrariwise correction of measurement signal falsification and set value of the trimming adjustment leads after the determination of the offset to the same dynamic lambda value as before the offset determination, since the trimming adjustment had previously corrected, with its set value and integral content, the error which developed due to the drifting of the measurement signal falsification. The trimming is much more frequently active than the offset determination, since the latter can be performed only under specific conditions of the operation of the internal combustion engine.
The invention thus has the advantage that the corrections of the trimming adjustment affecting emissions is preserved to the full extent even after an offset determination with any desired amount of compensation of the measurement signal falsification.
This has the advantage, furthermore, that now the set value of the trimming adjustment can be used without limitation for diagnosing the components of the exhaust gas cleaning system, since it especially permits obtaining information on the lambda probe situated ahead of the catalyst, because it is not influenced by component inaccuracies or temperature-related measurement signal falsification in the controller.
In a preferred embodiment, in the case of a lambda probe in which a current signal vanishing at lambda=1 is present which is converted by the controller to a voltage, the controller is switched to the test mode by being separated from the raw signal output of the lambda probe. Then no raw signal current flows into the controller. The voltage put out by the controller as the measurement signal represents the actual value of the measurement signal falsification. This test mode is activated either in lambda-1 phases, e.g., in idling operation, or sufficient time must be allowed for the said build-up.